It has been long recognized that niobium monoxide (NbO) has some unusual electrical properties that make it well-suited for the manufacture of electronic capacitors. For example, it is of much lower flammability than equivalent tantalum powders, is less costly than tantalum, and has a much larger potential supply than tantalum. However, niobium monoxide capacitor powders require high levels of purity, with not only foreign elements such as iron and copper being deleterious, but other forms of niobium such as niobium metal, niobium dioxide (NbO2), niobium trioxide (Nb2O3) and niobium pentoxide (Nb2O5) being potentially harmful as well. In order to be useful in a valve application, the niobium monoxide should be in a finely divided form, i.e., a fine powder or agglomerates formed from small particles, typically 1–2 microns in diameter, or finer. In order to meet these prerequisites, the electronics industry has produced niobium monoxide by reacting niobium pentoxide or niobium dioxide (possibly pre-reduced from the pentoxide) with a metallic reducing agent under conditions in which the niobium oxides remain in the solid state. This allows the particle morphology of the original oxide to be preserved in the niobium monoxide.
In one embodiment of this process, niobium pentoxide is reacted at temperatures of approximately 1000° C. with finely divided metallic niobium in such stoichiometric proportions as to produce primarily niobium monoxide. In another embodiment, the niobium pentoxide or niobium dioxide is reacted with gaseous magnesium, similarly at temperatures of approximately 1000° C. This results in a “spongy” niobium monoxide-magnesium oxide mixture. After leaching the magnesium oxide, the resultant product is a high-surface area, agglomerated mass of niobium monoxide.
Because of the low processing temperatures used in these methods of producing niobium monoxide, there is inadequate opportunity to remove impurities in either the niobium oxide or the reducing agent feedstock. The purity requirements of the niobium monoxide dictate the purity required of the feedstock. The surface area requirements of the product niobium monoxide further dictate the particle size distribution and morphology of the niobium pent-or-di-oxide required for the process. These requirements severely limit the availability of suitable raw materials. In addition, because the reactions occur in the solid state, the reactions are sluggish, and often do not go to completion. The product contains some higher oxides of niobium, and often some niobium metal.
Thus, an object of the present invention is to produce niobium monoxide (NbO) powder of high purity and sufficient surface area to meet the requirements of NbO capacitors without the constraints of raw materials purity and particle size imposed by solid-state processes, and further to the use of such powders in the production of capacitors.